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  #1  
Old 03-02-2010, 10:42 PM
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Cool W126 Clock project

Hi all,


This is a decidedly non-car related thread. What I want to do is figure out a way to turn my extra W126 tachometer clusters (with the clock) and make them into some really cool desk clocks.

Getting the clock to work is easy, and I already have it sitting on my desk keeping perfect time. What I want to do is figure out what kind of signals drive the tachometer so that I can use a basic stamp or some sort of cobbled together driver for it so that I can turn the tach needle into the second hand of the clock.

Since 60 is right near redline, it will make a really cool desk clock once I figure out how best to drive the tach needle's d'Arsonval movement.

Of course I will share my findings here if anyone else wants to make a W126 desk clock.

So -- Anybody know the specifics of how that tach is driven and by what type of signal?

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1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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  #2  
Old 03-03-2010, 12:15 AM
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It's PROBABLY a 12 volt square wave, with increasing frequency moving the needle. Try a lower voltage first. But the needle won't spin around - maybe you were thinking of going from 0-5900 RPM (100 per second)?
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  #3  
Old 03-03-2010, 05:07 PM
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Yes, a d'Arsonval movement will not turn 360 degrees, and I wasn't thinking it would. Basically, I envision the needle climbing from 0-5900, then dropping back to zero again. It would be nice if I could get it to move in 1 second intervals, but we'll have to see what the limitations of the driver circuit are......


I will try the square wave first and see what I get, and of course I will report back.
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1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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  #4  
Old 03-03-2010, 05:13 PM
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how long will it take to fall from 5900 to zero or will you just have it go back to zero at a 1 sec per 100 rpm?
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  #5  
Old 03-03-2010, 05:46 PM
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Quote:
Originally Posted by TheDon View Post
how long will it take to fall from 5900 to zero or will you just have it go back to zero at a 1 sec per 100 rpm?
Good question, I was thinking that it would just drop to 0.

Without any electricity applied, if I pull the needle up to 6000 with my finger and let go, it takes about 1/4 second to drop to 0, but it bounces off the zero a bit and makes a faint click.

It may very well turn out that 5 second increments are more realistic, or that it won't drop to zero fast or quietly enough, we'll just have to see. Right now I am trying to find a signal generator with a square output. My '40s navy model only makes sine waves, but it is soooo sweet with its air capacitor sweep knob.
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1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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  #6  
Old 03-03-2010, 05:55 PM
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Betcha a 555 IC will provide the square wave signal. Somebody with a bit more skill can likely give you the number for a ramp generator IC that will tell the 555 when to change frequencies (or maybe another 555 feeding the one driving the tach input).
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  #7  
Old 03-03-2010, 06:33 PM
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Quote:
Originally Posted by mramay View Post
Betcha a 555 IC will provide the square wave signal. Somebody with a bit more skill can likely give you the number for a ramp generator IC that will tell the 555 when to change frequencies (or maybe another 555 feeding the one driving the tach input).

I second this. 555's are great little signal generators
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  #8  
Old 03-03-2010, 06:35 PM
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mramay is right, a 555 will do it, I've done it with that IC aswell.... I dont recall the values of R1, R2, or C, or whether it was pulse width modulated or frequency modulated, but I do recall it took about 20 minutes of playing around with and eventually I found the right combo.


J
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  #9  
Old 03-03-2010, 06:35 PM
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Yeah, that sounds like a *much* cheaper way to accomplish the result. I have more 555s in my parts box than just about any other chip.

Still gotta find a signal generator with square wave.......
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1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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  #10  
Old 03-03-2010, 08:27 PM
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But with a microprocessor, you get fine control and adjustment (which can be tuned and stored), and less hardware There are plenty of good microprocessors that cost less than $1.50 in onezies that could do this job.
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  #11  
Old 03-03-2010, 08:41 PM
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So,


It looks like the easiest way to make a square wave with a 555 is just to use it in standard astable mode. This however is going to make changing the frequency quite difficult, so maybe the astable 555 can be used to generate a 60hz or 1hz clock pulse although it won't be terribly accurate for that without a great deal of tweaking. Besides, there are many better options for a clock pulse..... One could be taken from the clock itself, from 60hz AC power (like most older clock radios), or from a more stable quartz oscillator.

monostable operation would make more sense if one wishes to change the frequency, but the duration of the pulse is again set by a resistor and cap. One would have to have a short enough duration so that the pulses don't overlap at the higher frequencies, and the tach would have to be able to take pulses with a wide variance in duty cycle for that to work......


How exactly would you propose I employ said 555?

Also, a 555's output is limited to 200ma, so hopefully that would be enough to power the tach.............
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1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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  #12  
Old 03-04-2010, 06:58 PM
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This is my circuit, output driver seems OK with it. The 5K is a pot, you control the RPMs that way. You could get slick and get a microprocessor to vary the speed up and down, that would be cool. The tach does make an unnerving buzzing noise, though. You can't hear it over the engine when you're driving, but on a coffee table it will be pronounced.

I'd give it 2 minutes until the novelty wears off!

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Cool W126 Clock project-rpm-circuit.jpg  
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  #13  
Old 03-04-2010, 07:08 PM
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Hey, thanks for the circuit! I'll whip one up tonight or over the weekend and give it a shot.

If the thing makes a buzzing noise, I'll try to find a workaround such as a strategically placed capacitor.

If not, I found this:

http://wps.com/products/Model-13/index.html

and, wonderfully, he has posted the code for it here:

http://wps.com/J/Sources/clock.bas

What he has done is use a PIC controller to drive the d'Arsonval movement directly, and it looks like he has gotten excellent results. His clock is far more complex, using the single d'Arsonval to display time and date! What a cool project!

Anyway, if I can't get a silent driver for the existing tach circuit, perhaps I'll just remove it and drive the needle directly with a PIC controller, and steal some of his code.......

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1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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